The present invention relates to a method of producing an optical element molding die capable of molding an optical element and a method of producing an optical element.
In recent years, there has been used an optical element such as an extremely high-precision objective lens in the field of an optical pick-up device which is growing rapidly. When a material such as plastic or glass is molded to be the optical element like that stated above by the use of a die, products in a uniform shape can be manufactured quickly, and therefore, the molding by a die can be said to be suitable for a mass production of optical elements having the aforementioned uses. Since the dies are expendables, and are estimated to be damaged by an unexpected accident in this case, replacement of dies on a periodical basis or on a non-regular basis is considered to be necessary for molding high-precision optical elements. Therefore, it is necessary to prepare in advance a certain number of dies which are precise to a certain level, for the die for molding optical elements.
When a die is made through cutting work that employs a single crystal diamond tool or the like, the cutting work is time-consuming, and it is difficult to quarry out dies which are exactly the same in terms of a shape, which causes a fear of a difference of a product shape of an optical element between before and after replacement of dies, and increases a cost, resulting in a problem.
With the foregoing as a background, there is an attempt to manufacture a die by making an electroforming mold to grow for a base that has a base optical surface corresponding to an optical surface of an optical element. When a die making method employing the electroforming of this kind is used, it is possible to obtain, relatively easily, an optical element molding die having less dimensional dispersion, only by preparing an accurate base.
In the aforementioned method, however, the base is completed through a plurality of processes starting with cutting work for a material, which requires a region that serves as a reference for a series of processing in the course of the plural processes stated above. Since a reference for a design of an optical element is generally an optical axis, it is originally preferable to provide a mark that agrees with an optical axis. However, it is impossible to provide such mark on a base optical surface, because a shape of the optical surface is damaged.
With the foregoing as a background, an optical element of a certain kind used for an optical pick-up device is provided, for an improvement of aberration characteristics, with a diffractive ring-shaped pattern which is concentric with an optical axis of an optical surface. In this case, if a ring-shaped pattern corresponding to the diffractive ring-shaped pattern is formed on a base optical surface of a base member, it is considered that a position of the optical axis can be presumed accurately when the ring-shaped pattern mentioned above is used after electroforming processing. However, detecting the optical axis from the ring-shaped pattern and conducting processing with a reference of the ring-shaped pattern thus detected require an apparatus to read the ring-shaped pattern, and they are time-consuming. Further, when the base optical surface does not have the structure which is similar to the ring-shaped pattern corresponding to the diffractive ring-shaped pattern and is concentric with the optical axis, the method mentioned above cannot be used, which is a problem.
When the aforementioned method is not used, it is difficult to specify the position of the optical axis on the base optical surface in the course of making a die, because the base optical surface is covered with an electroforming mold by electroforming in the die making method employing the electroforming. As a result, it is difficult to process accurately a part of the electroforming mold on which a shape of the base optical surface has been transferred in the processing thereafter, or a part of another die, corresponding to the optical axis mentioned above. If the electroforming mold stated above is incorporated in the die under the condition that the position of the optical axis is inaccurate, deviation of the optical axis position and deviation of the position of the shape transferred by electroforming (for example, diffractive ring-shaped pattern) are caused, which makes it impossible to attain the original optical capacity as an optical element.